A process for desorbent recovery is described. Paraxylene production from selective adsorption systems creates raffinate and extract intermediate product streams comprised of desorbent and feed component mixtures. The desorbent can be characterized as either light or heavy to indicate its boiling point relative to the feed constituents. A desorbent recovery system is required to separate the desorbent for recycle. Recovery of light desorbents such as toluene requires energy intensive distillation where the desorbent is recovered as an overhead distillate product.
Toluene has a relatively low dew point temperature compared to a heavy desorbent with higher molecular weight. Therefore, higher condensing pressures are required to meet typical heat recovery temperature approaches within an aromatics complex. Higher pressure column operation is inherently less energy efficient as relative volatilities decrease, as well as more capital intensive as equipment becomes thicker and heavier.
The current approach to desorbent recovery employs a high pressure raffinate column. Operating pressure is high enough to utilize the overhead vapor condensing stream to provide heat input to low pressure column reboilers, including the extract column reboiler. This is a well-established practice yet an unacceptably large amount of heat from desorbent recovery is ultimately rejected to the atmosphere. Therefore, a means of reducing the desorbent recovery costs in a light desorbent system is needed.